To ensure that metal components possess the desired mechanical, physical, and chemical properties, heat treatment processes are often essential in addition to selecting appropriate materials and various forming techniques. Steel is the most widely used material in the machinery industry due to its complex microstructure, which can be controlled through heat treatment. Therefore, heat treatment of steel is a primary focus in metal heat treatment.

Furthermore, metals like aluminum, copper, magnesium, titanium, and their alloys can also have their mechanical, physical, and chemical properties altered through heat treatment to achieve different performance characteristics.

Heat treatment generally does not change the shape or overall chemical composition of the workpiece; instead, it modifies the internal microstructure or alters the surface chemical composition to impart or enhance the service properties of the component. Its characteristic feature is the improvement of the internal quality of the workpiece, which is typically not visible to the naked eye. The function of heat treatment is to enhance the mechanical properties of materials, eliminate residual stresses, and improve the machinability of metals.

Common Heat Treatment Methods for Parts

Chemical Heat Treatment

Chemical heat treatment involves using chemical reactions, sometimes combined with physical methods, to alter the surface chemical composition and microstructure of steel components. After chemical heat treatment, the steel components can be regarded as a special composite material. The main purpose is to improve wear resistance, fatigue strength, corrosion resistance, and high-temperature oxidation resistance. Chemical heat treatment methods include carburizing, nitriding, boriding, sulfidizing, aluminizing, chromizing, siliciding, carbon-nitrogen co-diffusion, oxynitriding, thiocyanate co-diffusion, and multi-component diffusion processes like carbon (nitrogen) titanium coatings.

Contact Resistance Heating Quenching

The principle of contact resistance heating quenching is to pass low-voltage current through the contact resistance between the electrode and the workpiece, rapidly heating the surface of the workpiece, which then cools quickly through its own thermal conduction. This method has the advantages of simple equipment, ease of operation, and good automation, resulting in minimal distortion of the workpiece. It does not require tempering and significantly enhances the wear resistance and scratch resistance of the workpiece, although the hardened layer is relatively thin (0.15–0.35 mm) and exhibits poor uniformity in microstructure and hardness. This method is mainly used for surface hardening of cast iron machine tool guides and has limited application.

Electron Beam Heat Treatment

Electron beam technology has been in use for over 20 years and is widely applied in metal welding and cutting processes. Electron beam heat treatment is a new technique that utilizes high energy density electron beams for surface hardening. The electron beam is emitted from a heated cathode (filament) through a high-voltage annular anode, focused into a beam that strikes the metal surface, achieving heating. The heating depth of the treated parts depends on the acceleration voltage and the density of the metal; for example, at 150 kW power, the theoretical heating depth in iron is 0.076 mm and in aluminum is 0.178 mm. Electron beam heat treatment has a fast heating rate, with austenitization time of only a fraction of a second, resulting in very fine surface grains, higher hardness than conventional heat treatment, and excellent mechanical properties.

Electrolytic Heating Quenching

Electrolytic heating quenching is a metal heat treatment process that changes the mechanical properties of the surface layer through heating and cooling the surface of steel components. Surface hardening is the main focus of surface heat treatment, aimed at achieving a hard surface layer and favorable internal stress distribution to improve wear resistance and fatigue resistance of the component. In electrolytic heating surface hardening, a direct current (150–300 V) is passed through an electrolyte, causing ionization and resulting in a conductive phenomenon. Hydrogen is released at the cathode, and oxygen at the anode. A hydrogen gas film forms around the cathode, increasing resistance and generating a large amount of heat, which heats the cathode. During quenching, the workpiece submerged in the electrolyte is connected to the cathode, while the electrolyte tank is connected to the anode. When the power is turned on, the submerged part of the workpiece is heated (reaching quenching temperature in 5–10 seconds). After power off, the workpiece can cool in the electrolyte or be transferred to a separate quenching tank. Various electrolytes are suitable for surface hardening, with a 5%–18% sodium carbonate solution being the most commonly used, provided the temperature does not exceed 60°C; otherwise, the hydrogen gas film becomes unstable, affecting the heating effect.

Laser Heat Treatment

Laser hardening involves using lasers to heat the material's surface above the phase transformation point, causing the austenite to transform into martensite as the material cools, thereby hardening the surface. Laser hardening of gear teeth involves high heating and cooling rates, resulting in short process cycles without the need for external quenching media. This method offers unique advantages, including minimal distortion of the workpiece, a clean working environment, no need for post-processing like grinding, and the size of the treated gears is not limited by the heat treatment equipment. Due to its high power density and rapid cooling speed, laser hardening is gradually replacing traditional processes like induction hardening and chemical heat treatment in many industrial applications, especially for parts with high precision requirements.

Vacuum Heat Treatment

Salt bath quenching is becoming outdated due to environmental limitations. Vacuum heat treatment is a new technology that combines vacuum technology with heat treatment. The vacuum environment refers to atmospheres with pressures below one atmosphere, including low, medium, high, and ultra-high vacuum. Vacuum heat treatment also falls under atmosphere-controlled heat treatment. The development and refinement of vacuum heat treatment technology have led to its widespread adoption, characterized by the absence of oxidation and decarburization, resulting in clean and bright surfaces post-quenching, high wear resistance, no pollution, and a high degree of automation. Commonly used vacuum heat treatment technologies in industrial production include vacuum annealing, vacuum degassing, vacuum oil quenching, vacuum water quenching, vacuum gas quenching, vacuum tempering, and vacuum carburizing, making it one of the most prevalent heat treatment techniques in workshops.

Induction Heat Treatment and Ion Nitriding Technology

Induction heat treatment is widely used in industries such as automotive manufacturing, construction machinery, and petrochemicals due to its efficiency, energy savings, cleanliness, and flexibility. Nearly 40% of automotive parts can be treated using induction heat treatment, including crankshafts, gears, universal joints, and half shafts. Induction heating allows for many products to be processed on fully automated or semi-automated production lines, improving product quality consistency, reducing labor intensity, and enhancing the work environment. The fastest development in this area is in induction heating power supplies, where outdated electronic oscillation tubes have been replaced by fully transistorized systems. Microcomputer-controlled transistors allow for stable and precise regulation, significantly reducing interference from power grid harmonics.

Use of New Surface Strengthening Technologies and Promotion of Nitrogen-Based Atmosphere Heat Treatment

Traditional surface treatment methods for tools were limited to outdated techniques such as steam treatment and oxynitriding, typically only improving tool lifespan by 30% to 50%. Since the 1980s, China has independently developed and introduced technologies like QPQ salt bath composite treatment and PVD titanium oxide coatings. The former can stabilize and extend tool lifespan by 2 to 3 times with simple equipment and low costs, making it particularly suitable for standard tooling. The latter can increase tool lifespan by 3 to 5 times, suitable for various precision and valuable gear tools. Nitrogen-based atmospheres are used to protect heat treatment and chemical heat treatment, enabling oxygen-free decarburization and reducing internal oxidation defects, thereby enhancing the quality of chemical heat treatment.